Reorganize core library docs.

doc/site/core/string.markdown

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+^title String Class
+^category core
+
+A string of Unicode code points stored in UTF-8.
+
+### **contains**(other)
+
+Checks if `other` is a substring of the string.
+
+It is a runtime error if `other` is not a string.
+
+### **count**
+
+Returns the length of the string.
+
+### **endsWith(suffix)**
+
+Checks if the string ends with `suffix`.
+
+It is a runtime error if `suffix` is not a string.
+
+### **indexOf(search)**
+
+Returns the index of `search` in the string or -1 if `search` is not a
+substring of the string.
+
+It is a runtime error if `search` is not a string.
+
+### **startsWith(prefix)**
+
+Checks if the string starts with `prefix`.
+
+It is a runtime error if `prefix` is not a string.
+
+### **+**(other) operator
+
+Returns a new string that concatenates this string and `other`.
+
+It is a runtime error if `other` is not a string.
+
+### **==**(other) operator
+
+Checks if the string is equal to `other`.
+
+### **!=**(other) operator
+
+Check if the string is not equal to `other`.
+
+### **[**index**]** operator
+
+Returns a one character string of the value at `index`.
+
+It is a runtime error if `index` is greater than the length of the string.
+
+*Note: This does not currently handle UTF-8 characters correctly.*